1. Field of the Invention
The present invention relates to a semiconductor fabrication, and more particularly, to a method of forming a pattern for a semiconductor device.
2. Description of the Prior Art
As sizes of semiconductor devices are designed as small as possible to catch up with the tendency of miniaturization, the patterning process becomes more and more important nowadays. In a traditional patterning process, the photoresist is patterned by one photolithography process, and thereafter serves as an etching mask for etching the underlying material layer. However, the miniaturization is limited by the exposing ability of current lithographic tools in traditional patterning process, because every lithographic tool has its critical exposure resolution of exposing gaps between lines and between spaces.
Accordingly, another pattern forming approach including two photolithography processes and one etching process is adopted when the pitch (the distance of centers of two neighboring structures) is smaller than 155 nanometers (nm). Please refer to FIG. 1, which is a schematic diagram illustrating the pattern forming approach including two photolithography processes. As shown in FIG. 1, one target pattern 10, which is designed for a semiconductor device (not shown), is divided into two partial patterns 12. Thus, the pitch of the target pattern 10 can be much smaller than the pitches of the partial patterns 12, while the pitches of the partial patterns 12 should relate to the critical exposure resolution of the lithographic tool (not shown). As a result, the pitch of the target pattern 10 can be superior to the critical exposure resolution of the lithographic tool.
However, it is still some problems for applying the concept to various patterning processes in practice. When the pitch of contact holes is lower than 140 nm, the above-mentioned two-exposures-then-one-etching method fails because it is beyond the limitation of the current lithographic tools during the second exposure, and consequently contact holes lower than 140 nm pitch cannot be produced.